The ornamental design for a pivotableshelf, as shown and described.
FIG. 1 is a bottom perspective view of the pivotable shelf;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines shown in FIGS. 1 and 7 are for the purposes of illustrating the environment of the article only and form no part of the claimed design.